1. Field of the Invention
This invention relates to glass seals for beta-alumina in electro-chemical cells and other energy conversion devices containing sodium and to glasses for use in such seals.
2. Prior Art
In cells, such as for example sodium-sulphur cells, and other devices (e.g. sodium-sodium thermo-electric generators) where beta-alumina ceramic is used as a solid electrolyte through which sodium ions can pass, it is necessary to seal the beta-alumina in the cell structure. Sodium sulphur cells utilise a molten alkali metal and have to operate at elevated temperatures where the electrode materials are liquid. Seals necessary to seal these electrode materials within the cells have therefore not only to withstand highly reactive materials at these elevated temperatures but are also subjected to temperature cycling. As a typical example, a sodium sulphur cell might contain a beta-alumina electrolyte tube closed at one end and having sodium on one face of the tube, preferably the outer face, and the sulphur/polysulphides on the other face. The cell has to be sealed to prevent escape or mixing of these materials and a number of proposals have been made for the sealing of such cells. Compared with most metals, ceramic materials are generally weak, particularly in tensile strength and it is necessary therefore in any seal for such a cell to ensure that the ceramic material is not overstressed.
It is well-known in a sodium sulphur cell to provide a beta-alumina electrolyte tube with an alpha-alumina tubular extension at its open end so that the end portion is ionically insulative. The alpha-alumina and beta-alumina have similar coefficients of thermal expansion and the alpha-alumina extension can be secured to the beta-alumina with a glass seal.
The present invention is concerned more particularly with such glass seals. The use of glass seals is described, for example in U.S. Pat. Nos. 3,928,071, 3,826,685 and 3,868,273. These references however do not discuss the glass composition. It is convenient, in sodium sulphur cells and similar electro-chemical cells to use a glass as a bonding agent between ceramic materials or between a ceramic material and a metal member. In a sodium sulphur cell, the closure may be effected by sealing the ceramic electrolyte material to a closure member or to the housing. This may readily be done with a glass seal. The alpha-alumina extension then has to be sealed to the housing or to the closure member. The closure member may be a part of a current collector. Thus glass may be employed, in sealing a cell, as a bond between solid electrolyte material, e.g. beta-alumina ceramic, or an insulating ceramic, e.g. alpha-alumina, and a metal component or components such as a current collector, an intermediate component, or an external housing. The glass-to-metal bond is formed by a reaction between the glass and an oxide layer on the metal.
Both for sealing alumina to metal and for sealing beta-alumina to alpha-alumina, the glass employed must be a sodium-resistant glass. The metal material has to be chosen in accordance with both mechanical and chemical requirements. In particular, it must resist attack by sodium at elevated temperatures. It is the practice in sodium sulphur cells to use mild steel or stainless steel for the housing, in contact with the sodium.
In particular, use has heretofore been made of a glass, commonly known as BAB glass. As is described in U.S. Pat. No. 3,275,358, this glass, which has a composition by weight of about 40% B.sub.2 O.sub.3, 25% Al.sub.2 O.sub.3 and 35% BaO can be used for providing a fused hermetic seal around lead wires of tantalum and other similar metals. Expressed in molar percentages, the composition of this glass is about 54.8% B.sub.2 O.sub.3, 23.4% Al.sub.2 O.sub.3 and 21.8% BaO.
This BAB glass however does not fully meet the difficult service conditions of seals for sodium-sulphur cells, particularly alpha to beta alumina seals, which place a severe demand on the performance of the sealing glass used. The required characteristics of such a glass in terms of its physical and chemical properties are many and sometimes conflicting and so the development of a glass composition approaching the desired behaviour is of necessity a complex process. It must produce a reliable hermetic seal and hence must wet both the alpha and beta alumina. The glass should be chemically compatible with cell materials at cell operating temperatures. The thermal expansion characteristics of the glass and the other seal components must be compatible to give low seal stresses over the whole working temperature range and lifetime of the cell.
In general, most stable glasses will satisfactorily wet both alpha and beta alumina and so could potentially satisfy the first criterion. The requirement as the chemical compatability considerably constrains the choice of glasses to the use of the alumino-borate glasses because of their known good sodium resistance. The requirements about thermal expansion characteristics are not satisfactorily met by BAB Glass.
It is necessary to consider in further detail the problems of seal stresses. The theory and practice of glassed seals, notably glass-to-metal seals, is very well established: See, for example, "Glass-to-Metal Seals" by J. H. Partridge, Soc. for Glass Technology, Sheffield 1949 and "Glass-to-metal Seals" by A. W. Hull and E. E. Burger, Physics 5. 384-405 1934. One of the most important factors contributing to successful seal performance is the close thermal matching of the seal components. Differential thermal strain causes seal stresses which, if sufficiently high, can result in seal failure.
At high temperatures when a seal is made, the glass is fluid and wets the ceramic components. On cooling, the viscosity of the glass rapidly increases and eventually it becomes so high that the glass behaves as a rigid material. It is convenient for analysis to idealise the transition between the fluid and rigid regions as occurring suddenly at a single temperature called the set point, even though in reality the change takes place over a small range of temperatures. Above the set point any thermal stresses developed in the glass are relieved by viscous flow. However, at the set point the glass becomes rigid and mechanically constrains the seal components, so that on further cooling, stresses are developed due to the subsequent differences in thermal strain characteristics.
A convenient graphical construction to assess the magnitude of the differential strain is shown in FIG. 1 of the accompanying drawings where the expansion/contraction characteristics (dimension change plotted as ordinate against temperature as abscissa) of the seal components are plotted and the characteristics are vertically displaced so as to intersect the set point. Assuming that the seal components are elastic, the stresses generated at any temperature are proportional to the vertical separations of the two curves in FIG. 1, and so the stress-temperature characteristic has the shape shown in FIG. 2, in which the stress is plotted as ordinate against temperature as abscissa.
For sodium-sulphur cells, the properties of the ceramics to be joined are effectively fixed. The main seal stresses in the glass are material dependent only and are not greatly influenced by seal geometry except at points close to the seal edges. The composition of the sealing glass does however have an important effect on the seal stress characteristics.
Because of its brittle nature, glass has a much higher strength in compression than in tension. Thus, for good seal strength, a high crossover temperature (see FIGS. 1 and 2) is desirable so that the seal stresses are compressive both at room temperature and operating temperature (typically 300.degree.-400.degree. C.). This is not the only criterion for satisfactory seal stresses because seal cracking and failure can also occur if the room temperature stress is too highly compressive.
All glasses have non-linear thermal strain characteristics and these are such that a sealing glass will always be in tension at high temperatures. By using a glass with a high set point, it is possible to have both a high crossover temperature and a moderate room temperature compressive stress as is shown in FIG. 3, which is a diagram similar to FIG. 2 but for two further different glasses.
Seals made with BAB glass have a generally tensile seal stress characteristic and tend to crack easily in thermal cycling of sodium sulphur cells. An additional disadvantage of this glass is a high densification rate. Densification, sometimes called stabilisation, is a process of molecular reordering of a glass at temperatures below the glass transition point, resulting in a gradual shrinkage of the material. When constrained within the seal components, the sealing glass is thereby subjected to increasing tensile stresses, which can cause seal failure if they grow sufficiently large. It might appear that improvement could be obtained by altering the relative proportions of the three constituents in this glass. However, as will be apparent from consideration of the data given by C. Hiramaya, J. Amer. Ceram. Soc. 1961, 44(12), 602-6 on this ternary system that the particular composition or BAB glass is on the edge of the glass forming region of the phase diagram of this three-phase system and it has not been found possible to satisfactorily lower the expansion coefficient of the glass of this ternary system by altering the proportions of the components.